As is well known in the gas turbine engine art it is typical for the aircraft fuel pumping system to utilize a centrifugal boost stage and a high pressure gear stage, mounted in series flow relationship, so as to supply fuel to the fuel metering control and to the engine variable geometry actuators. Another typical aircraft installation would include two separate pumps, operating in parallel flow relationship, to supply fuel separately to the fuel metering control and to the variable geometry actuator.
It is also well known that centrifugal pumps afford improvements over the well known positive displacement types insofar as they are lighter in weight, smaller in size for the same flow capacity and more durable. They are also capable of operating dry in the event the system being supplied with fuel is inoperative. As for example, such pumps have heretofore been utilized with augmentors or afterburners sometimes used on military aircraft.
However, whenever centrifugal pumps have been utilized to supply the main engine fuel flow, a positive displacement stage, notably a gear or vane type, has been incorporated inasmuch as the centrifugal pump cannot provide sufficient pressure at the start-up mode to operate the fuel control. Thus, a typical installation would deactivate the positive displacement stage as soon as the engine was up to sufficient speed, say idle, to operate the centrifugal pump. Hence, the vane or gear pump would be decoupled and would remain inoperative until the next engine start or restart cycle. In some installations the decoupling would be a clutch arrangement and in others it would be a vane retraction system, or the like. Whether the one or other is being employed, such systems are not only complex but generally contribute to the overall weight of the pumping system.
This invention addresses two significant problems associated with the high speed centrifugal pump, which are namely (1) The high speed centrifugal stage is less able to accommodate the two-phase vapor/liquid (V/L) flow which results when dissolved air is released into the fuel upstream of the pump inlet, as can occur during aircraft climb operation; and, (2) The centrifugal stage cannot generate sufficient pressure at engine cranking speeds to satisfy the requirements of the fuel metering control and variable geometry actuator, necessitating the incorporation of a positive-displacement type starting stage and an automatic disengagement mechanism. The positive-displacement stage is also necessary to provide a dry lift capability to enable the pump to reprime itself after any inadvertent fuel mismanagement incident. The starting stage must be large enough to provide full starting flow, adding to the weight of the pump, and the automatic disengagement mechanism reduces overall pump reliability.
In accordance with this invention the fuel pump system comprises a high-speed centrifugal main stage to supply fuel to the fuel metering control, a small gear-type hydraulic stage to supply fuel to the engine variable geometry actuator and to the fuel metering control servos, and a low-speed centrifugal stage to supply boost pressure to the centrifugal main stage and the gear hydraulic stage.
The use of the centrifugal pump in the concept as described herein overcomes the following shortcomings of the heretofore known pumps:
1. The high-speed centrifugal main stage retains the weight and durability advantages inherent with this configuration, but is now capable of providing engine starting fuel flow as well, since it is not required to provide the high pressure necessary to power the fuel metering control servos and engine variable geometry actuator(s).
2. The low-speed centrifugal boost stage provides sufficient additional two-phase flow handling capability to satisfy installed engine V/L requirements. Because the high-speed centrifugal stage can accommodate a modest V/L condition by itself, the low-speed centrifugal boost stage is smaller than that required for a conventional gear pump.
3. The gear hydraulic stage provides the high fuel pressures necessary to power the fuel metering control servos and the engine variable geometry actuator(s); however, because the flow required for these purposes is less than that required for the engine burners, this stage is smaller and lighter than the positive-displacement type stage that would otherwise be required for engine starting. The hydraulic stage also provides the dry lift capability required for fuel system repriming, by means of a vent valve provided either in the pump or in the downstream metering or actuating control system.
Inasmuch as the pumping system of this invention utilizes the high speed centrifugal pump solely for the main fuel metering system the centrifugal boost stage and the gear hydraulic stage operate at lower speeds than that of the high-speed centrifugal main stage. The gearing necessary to drive the three stages at the required speeds can be located either in the engine gearbox or in a combined pump assembly. If the gearing is located within the pump assembly, it may be lubricated either with engine oil from the gearbox, or with fuel passing through the pump.